Smaller and higher aspect ratio features, such as vias, trenches and contact holes, in semiconductor manufacturing impose greater requirements on semiconductor processing equipment. For example, coating contacts on the bottoms of such features with liners and filling the features with conductive films using certain preferred physical vapor deposition (PVD) processes requires the achievement of a high degree of directionality in movement of the material being deposited toward the substrate. Smaller and higher aspect ratio features require greater directionality. To effectively coat contacts, for example, on the bottoms of narrow high aspect ratio holes on the surface of a substrate, it is necessary for the particles of coating material to move at angles to the normal that are not substantially larger than the angular openings of the features. Otherwise, excessive deposits on the upper sides of the features or a closing of the mouth of a feature will result.
A sputter coating process is typically carried out by placing a substrate and a target of high purity coating material into a vacuum chamber filled with an inert gas such as argon or a reactive gas such as nitrogen and creating a plasma in the gas. The plasma is typically generated by maintaining the target, either constantly or intermittently, at a negative potential, so that the target functions as a cathode to supply electrons that excite the gas in the chamber and form a plasma adjacent to the target surface. The plasma generation is usually enhanced with a magnetron cathode assembly in which magnets behind the target trap electrons at high density over the surface of the target where they collide with atoms of the process gas, stripping electrons from atoms of gas to produce positive ions. The gas ions accelerate toward the target, which is negatively biased, to collide with the target surface and eject from the target surface atoms and atomic clusters or particles of target material, as well as secondary electrons, which play a role in sustaining the plasma.
In conventional sputter coating, the large majority of the ejected atoms of target material are neutral in charge and propagate through the vacuum space in various directions with some striking the substrate, to which they adhere to form a film. The directions of travel of the ejected particles from the target surface follow a somewhat broad statistical distribution of angles to the target surface. Various schemes have been used to cause the propagating particles to move in straighter lines toward and normal to the substrate surface. In Ionized Physical Vapor Deposition or IPVD, coating material is sputtered from a target using magnetron sputtering, other conventional sputtering or evaporation techniques, and then the directionality of the particles is improved by ionizing the particles so that they can be electrostatically accelerated or otherwise electrically steered in a direction toward and normal to the substrate.
In IPVD, additional or secondary plasma is created in the space within the chamber between the target or source of the material and the substrate. The particles of sputtered material passing through this space collide with electrons or metastable neutrals of the ionized process gas, which tend to strip electrons from the atoms of the sputtered particles leaving the particles positively charged. Those positive ions of sputtered material that are positively charged are capable of being electrically accelerated toward the substrate, for example, by application of a negative bias to the substrate. The effectiveness of the IPVD process in normalizing the direction of coating particles at the substrate is proportional to the fraction of ionization of the sputtered material produced by the secondary plasma.
Obtaining a high ion fraction of sputtered material requires the secondary plasma to have a high electron density. Loss of electrons from the secondary plasma into the main plasma at the target, or into chamber structure such as walls or shields, can cause a substantial reduction in the effectiveness of the secondary plasma to ionize sputtered material and can result in the extinguishing of the secondary plasma. It is important to minimize the depletion of electrons from the secondary plasma and to otherwise produce a high ionization fraction of sputtered material in IPVD processing.
In addition, structure such as walls or shields that bound a secondary plasma is in direct contact with the secondary plasma in a region called the sheath. The sheath width depends in part on the potential difference between the secondary plasma and this structure. Where the structure is electrically grounded, the typical sheath width is a few electron Debye lengths of about 0.14 mm, for example, where the electron density and temperature are about 10.sup.10 cm.sup.-3 and 4 volts, respectively. However, if a negative DC potential is allowed to exist on this structure, it has the effect of attracting positive ions from the plasma due to an increase in the width of the plasma sheath, which thereby reduces the effectiveness of the plasma in producing a high ion fraction of the sputtered material. Where it is necessary to facilitate the coupling of energy into the secondary plasma, such as from a peripheral coil to form an inductively coupled plasma, the plasma surrounding shields and other structure are electrically floating, which increases the tendency for electrons, which have a higher velocity than the positive ions in the plasma, to build up a negative DC charge on the shield or other structure. This causes the plasma sheath to encroach into the space desired for the secondary plasma.
Accordingly, there is a need for an IPVD apparatus and method that will provide a high ionization fraction of sputtered material, and particularly that will minimize the loss of electrons from the plasma that is provided for sputtered material ionization. Further, there is a need for an IPVD apparatus and method that will provide a high ionization fraction of sputtered material, particularly by avoiding an extension of the plasma sheath that surrounds the plasma provided for sputtered material ionization into the space of the secondary plasma.